Schönbrunn (Vienna) is 17 ha/42 acres and a top zoo regardless of size. In fact, I do consider it the single best zoo for families with smaller children since the only I personally rate higher (2x Berlin, Bronx, San Diego, etc) all are quite large, hilly and/or just too rich in species. Children and most ordinary zoo visitors get tired before seeing half. Schönbrunn is manageable in size, doesn't have any bad exhibits (some are average, but nothing below) and has pretty much all the ABC species, incl. less regular ones like giant panda and koala.

From a design perspective parts of Schönbrunn are unusual because of the relatively high number of protected buildings, but they've generally solved this quite excellently.